The present invention relates to a hydraulic torque converter.
A hydraulic torque converter is one type of device used as a hydraulic transmission for a road vehicle. Normally it is arranged between the engine and the gearbox, and includes an impeller vane wheel attached to the output shaft of the engine, a turbine vane wheel attached to the gearbox input shaft, and a stator vane wheel. All these three are maintained in a bath of transmission fluid, and the turbine vane wheel has a fluid circulation inlet which is disposed directly adjacent to the fluid circulation outlet of the impeller vane wheel and which receives the fluid ejected therefrom. The fluid flow out from the fluid circulation outlet of the turbine vane wheel is received by the fluid circulation inlet of the stator vane wheel, which deflects this flow and supplies it from its fluid circulation outlet back to the fluid circulation inlet of the impeller vane wheel.
The general structure of such a torque converter is that the fluid circulation substantially occurs between two toruses which are coaxial, and one of which is inside the other, the pattern of circulation being rather like that of a smoke ring. In this type of torque converter it has been considered necessary for maintenance of operational efficiency that the velocity component of the circulating fluid in a plane which includes the axis of the torque converter should be constant as the fluid passes round its path. In order to achieve this, a basic design condition of conventional torque converters has been as follows. If we consider a section of the abovementioned inner and outer toruses by a plane which contains the axis of these two toruses (any plane will do, since the torque converter is cylindrically symmetrical), in this plane the fluid circulates around an annulus defined by an inner closed curve provided by the intersection of the plane and the inner torus and an outer closed curve provided by the intersection of the plane and the outer torus. Considering a circle inscribed between these two curves and touching both of them, the requirement that the velocity component of the fluid in the plane of section should be constant means that the product of the radius of such a circle, and the distance of its center from the axis of the toruses, must be constant for all positions of the circle around the annulus. This condition has always heretofore been maintained as a basic design constraint for torque converters.
However, the present invention derives from the realization that perhaps this condition does not have to be applied strictly. The point is that in an actual torque converter the fluid flow is not altogether in the plane of section as described above. There is a certain amount of deflection of the fluid around the axis of the toruses, caused by the vanes. Consequently, if the conditions within the vane wheels of an actual torque converter are evaluated, the annular shape according to the simple and geometric condition above may require some modification.
Considering a simple vane arrangement as viewed end on, as seen in FIG. 1, if the vanes are of the same thickness throughout their length, it is clear that the width of the fluid passage formed between the vanes, which is expressed by the diameter G of circles inscribed between two adjacent vanes, increases from their inlet portion to their middle portion, and then decreases again from there on to their portion, and in fact this width corresponds to the curvature of the vanes, being greater, the greater is the curvature. When fluid is flowing along this fluid path, being an actual fluid rather than an ideal one, it cannot keep up with the speed changes necessary to maintain its flow in proper correspondence to the varying width of this flow path, and therefore, as illustrated in FIG. 1, in practice turbulence is created at the parts of the flow path which have a tighter curvature, and a loss of efficiency is caused. This problem is in general solved by the thickness of the vanes being made different at different portions, as shown in FIG. 2, so that the flow path formed between the vanes is of constant width. However, if it is desired to manufacture the vanes of the torque converter out of standard steel plate of constant thickness, in view of cheapness and ease of assembly, this solution cannot be adopted.
Now, for a hydraulic torque converter used in a vehicle transmission, the times when it is called upon to transform the torque are substantially limited to the times when the vehicle is either moving off from rest, or accelerating. At other times there is no need to convert or multiply the torque, and it is desirable that as far as possible the operation of the torque converter should be an operation of direct connection, with minimum slipping. That is, it is a desirable characteristic of such a torque converter that in the low-speed driving range a large amount of slipping should occur, and therefore the torque ratio should be comparatively high; while in the high-speed driving range the slipping between the impeller and the turbine should be as small as possible, and hence the torque ratio should approach unity. In accordance with this desirable characteristic, it has been proposed, in U.S. Pat. No. 4,044,556, which was assigned to the same assignee as the present application, to construct a torque converter of a non-conventional shape, wherein this high-speed range slipping is minimized. In this proposed torque converter the slippage at high speed is so low that its operation approaches to direct connection, and therefore, if a lock-up clutch is further incorporated into the system, when this clutch is engaged or disengaged, very little torque shock is caused. Thus the above-identified patent proposes a torque converter which is particularly suitable for use with a lock-up clutch.
The non-conventional shape of the torque converter of the above-identified invention is as follows. Considering the annulus defined by a section by a plane which contains the axis of the two toruses, as defined above, which is delimited by an inner and an outer closed curve, the invention contemplated to provide a torque converter wherein this annulus was compressed in the axial direction, so that the dimension of the outer closed curve in the direction parallel to the axis of the toruses was smaller than its dimension in the direction perpendicular to that axis. Such a torque converter can schematically be seen in FIG. 3. In the above-identified prior patent, the ratio of the axial to the radial dimension of the outer delimiting closed curve is defined as being substantially in the range 0.64-0.8; and, further, the ratio of the distances from the axis of the toruses of the innermost portion of the outer delimiting curve, and of its outermost portion, is defined substantially to be in the range 0.4-0.33; and, finally, the ratio of the total cross-sectional area of the output of the impeller vane wheel (which is an area of an annular shape, in a plane perpendicular to the axis of the toruses) to the area of the circle outlined around the axis of the toruses by the outermost portion of the outer delimiting curve is defined substantially to be in the range 0.18-0.23. In such a torque converter it is found that the slippage rate at high speed is very low, and in fact such a torque converter is particularly suited for use in a transmission which includes a lock-up clutch.
However, it is found that in such a torque converter according to the aforementioned prior invention the lengths of the fluid circulation flow paths within the impeller vane wheel, the turbine vane wheel, and the stator vane wheel are smaller than in a conventional torque converter. Since the angle through which the flow velocity of the fluid has to be altered is the same, this means that the curvature of the vanes must be tighter, particularly at portions of the belowmentioned point B and in its vicinity. Accordingly, the above-outlined problem of turbulence at the portions of the fluid path where the flow direction of the fluid is changing sharply is aggravated.